How can I use pencil style strokes in frestyle?
Line Style Texture

That effect is achieved by overlaying a texture to the freestyle stroke.
Since 2.71 version of Blender, there is an option that let you easily control this aspect of the rendering whitout relying on composting. You can find further information in the Blender 2.71: Freestyle NPR Rendering release notes page.

Basically you should go to the Render Layer tab and enable the Use Texture option in the Freestyle Line Style



Then go to the Texture tab and edit the relative Linestyle Texture.



A better example is provided in the notes themselves: you can dowload it here. In this case you'll see an example of usage of a charcoal texture that led to results very similar to the ones you mentioned: